Heat-treating method



Oct. 16,1928. Y 1,687,656

W. J. BROWN mm TREATING ammo Filed m 17, 1926 a: s5 ,2: I \Q; $093 WITNESSES:

INVENTOR h alrerJBrawn A ATTORNEY Patented 0a. 16, 1928.

UNITED STATES PATENT OFFICE.

WALTER JOHN BROWN, OF DAVENPORT PARK, ENGLAND, ASSIGNOR TO WESTING- HOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYL VANIA.

HEAT-TREATIN G METHOD.

Application filed July 17, 1926, Serial No. 123,189, and in Great Britain July 23, 1925.

This invention relates to heat treatment of materials such, for example, as is utilized to eiiect surface hardening.

In surface hardening and like heat treatment of materials, particularly metals, great diificuities are frequently experienced owing to distortion and strains set up in the interior sections thereof. The object of this invention is to overcome these difficulties by heating the surface of the material without very greatly raising the temperature of the inte- According to this inventionfamethod of heat treating an object consisting of electrically conducting material comprises subjeeting the object of an alternating electro-magnetic held of such high frequency that heat is developed substantially at the surface only of the object. After the surface has been raised, in this manner, to the desired temperature the object is quenched before the interior parts thereof have reached an undesirably high temperature.

In order to attain the greatest possible difference between the temperature of the outer surface and that of the interior, it is necessary to minimize the heat losses from the outer surface. These losses occur by conduction to the interior of the object and by radiation and convection from the outer surface. It is necessary, therefore, to raise the temperature as rapidly as possible in order to reduce the time during which these losses occur. Radiation and convection losses may, in some cases, be further reduced by raisin the surroundings of the object to, or nearly to, that temperature to which it is desired to heat the surface of the object. In order that the interior parts of the object may not reach an undesirably high temperature, the object may be quenched while it is still being subjected to the alternating electromagnetic field. By this means also .it is ensured that the surface of the object is cooled from its maximum temperature with extreme rapidity.

In the case of surface hardening magnetic materials, such as steel, the correct temperature for quenching may be determined by the sudden change in the strength of the alternating current producing the alternating electromagnetic field, or by the corresponding primarily by the surface of the material losing its magnetic properties at a certain temperature.

In the accompanying drawings,

Fig. 1 illustrates diagrammatically the electrical apparatus required for surface hardening an object, such as a pinion;

Fig. 2 is a diagrammatic view, in central vertical section, of a simple form of apparatus for practicing the invention, and

Fig. 3 illustrates, by means of a central vertical section, another form of apparatus for practicing the invention.

Like reference numerals indicate like parts, where applicable, throughout the several figures of the drawings.

Referring now to Fig. 1, a pinion 11 is disposed within an inductance coil 12 comprising a plurality of convolutions of copper tube, preferably of square section. Water is circulated through the tubular conductor in order to keep it relatively cool. The terminals of the inductance coil are connected quency choke coil 16 is connected. between that plate of the condenser 15 which is remote from the anode and the cathode of the valve 14, and is coupled electromagnetically to an inductance device 17, which is connected in.

series relation with a grid leak 18 and condenser 19 between the grid and cathode of the valve 14. Energy is supplied to the anode circuit of the valve 14 from a generator 20 of high-voltage current through a high-frequency choke coil 21. The cathode of the valve 14 is heated by means of a battery or other means not shown in the drawin The arrangement is such that the valve 14 maintains electrical oscillations in the circuit formed by the inductance coil 12 and condenser 13. the values of which mainly determine the frequency of the oscillations.

Referring now to Fig. 2, the pinion 11 is axially disposed within the inductance coil 12 upon a support 2, A bath 22, containing water, oil or other desired quenching fluid, is movably arranged below the inductance coil 12, so that it may be rapidlyraised to immerse thepinion 11 in the cooling fluid.

In operation, high-frequency currents are,

passed through the inductance coil 12 by means of a generating apparatus, such as is illustrated in Fig. 1, to induce circulating currents in the pinion 11. The frequency of the high-frequency alternating currents is so chosen that the circulating currents are confined substantially to the periphery of the inion. If the frequency chosen is sufliciently high, these circulating currents will, in fact, follow the contour of the teeth of tire pinion, with the result that the tooth faces are raised to a suitable high temperature while the remainder of the pinion may remain relatively cool, provided that energy is supplied at a sufliciently high rate.

As soon as the surface of the pinion 11 has reached a sufficiently high temperature, the bath 22 is raised so that the pinion 11 is immersed in the quenching fluid.

The current in the inductance coil 12 is not interrupted until the pinion has become submerged in the quenching fluid.v

By this procedure, the pinion may be quenched at its highest temperature, there being no time intervening between the heating operation and the quenching operation.

Instead of providing a bath 22 containing cooling fluid, a series of jets may be disposed to project cooling fluid between the convolutions of the inductance coil 12 upon the surface of the pinion 11. In this case, the flow of cooling fluid through the jets may be turned on as soon as the surface of the pinion 11 reaches the required temperature.

Referring now to Fig. 3, instead of the inductance coil 12 being constructed ofatubular conductor and cooled by the circulation of fluid therethrough, it is 'made of solid material and enclosed within a tube 25 of heatresisting material, such as silica, which is supported upon a block 26, also of heat-resisting material, within a lagged container 27. An orifice 29 is provided below, and concentric with, the inductance coil 12, and a tank 24 is disposed with an entrance registering with the orifice 29, so that the pinion 11 may be dropped downwards into the tank 24, which contains the desired cooling fluid.

In order to raise the temperature within the lagged container 27 up to, or nearly to, the temperature to which it is desired to heat the surface of the pinion 11, for the purpose of enabling the rate of temperature rise of the surface of the pinion 11 to be very large by minimizing the loss of heat from the surface of the pinion by radiation, convection and conduction, gas burners 28 are arranged to project through the sides of the container 27 so that the flames therefrom impinge upon the tube 25.

In operation, the gas burners are lit and the temperature of the tube 25 and inductance coil 12 is raised to nearly the temperature to i which it is desired to heat the surface of the pinion 11. Ilhe alternating current is then circulated through the inductance coil 12, and the pinion 11 is introduced and held within the inductance coil 12 until its surface reaches the required temperature, whereupon it is immediately dropped into the tank 24, which contains the quenching fluid.

The desirable frequency of the alternating magnetic field depends upon the size and shape of the object under treatment, upon the electrical resistivity and magnetic permeability of its substance, and upon the thickness of the outer skin to which it is desired to confine the generation of heat.

In the case of a right circular cylinder of reat length relative to its diameter, which is subject to an axial field, the frequency is given approximately by the formula 2-- where f== frequency in cycles per second resistivity in electro-magnetic.

4 units permeability in electro-magnetic units t= thickness in ems. of the outer skin in Whichit is desired that sub stantially the whole of the heat should be generated. The thickness of skin to which it is necessary to confine the eneration of heat depends on the depth to W IIIC-h it is desired to harden or otherwise heat treat the object. The hardening dept-h can be controlled in two ways. In the first place, the thickness of the heat-gencrating skin may be controlled, as just demonstrated, b suitable choice of frequency. In the secon place, the temperature gradient may be controlled by alteration in the rate of heating. By increasing the intensity of the alternating magnetic field, the rate of input of heat energy to the outer skin is increased, and the temperature gradient from the outside to the inside of the object is also mereased. In practice, it is necessary that the rate of energy input should be very high indeed in order that the temperature gradient may be sufficiently high. This condition involves the supply of a very large amount of alternating-current power to the inductor coil for a comparatively short period of time.

When ferrous materials are being treated it is frequently convenient to determine, by electrical means, the instant at which the object should be quenched. Ferrous materials in general are magnetic at low temperatures, but at a certain high temperature a sudden change in permeability occurs and their magnetic properties disappear. The disappear-' in the tuned circuit and by a marked decrease in the current supplied by the generator to said tuned circuit. Since the temperature at which the magnetic properties of a ferrous material disappear is frequently that temperature at which'it is desirable to quench the material for hardening, it is convenient to so organize the system that the change ofcurrent in the tuned circuit, i. e. the inductance coil 12, or the corresponding change in the input or output of the generator or converter supplying said current, either gives an indication that the object should be quenched or itself automatically releases the object or raises the quenching bath. Such a system may readily be constructed by making use of a relay connected, for example, in the circuit of the generator 20;

It will be appreciated that two embodiments only of the invention have been de scribed, by way of example, and that many other modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

I claim as my invention:

1. The method of heat-treating an object consisting of electrically conducting material which comprises raising the surrounding temperature nearly to that to which it is desired to heat the surface of the object, sub jecting the object to an alternating electromagnetic field of such frequency that heat is developed substantially at the surface only of the object, and quenching the object before the interior thereof has reached an undesirably high temperature.

2. The method of heat-treating an object consistin of magnetic material which comprises raising the surrounding temperature nearly to that to which it is desired to heat the surface of the object, subjecting the object to an alternating electromagnetic field of such frequency that heat is'developed sub stantially at the surface only of the object, and quenching the object as soon as the magnetic properties of the material materially change.

3. The method of heat-treating an object consisting of magnetic material which comprises raising the surrounding temperature nearly to that to which it is desired to heat the surface of the object, subjecting the object to an alternating electromagnetic field of such frequency that heat is developed substantially at the surface onl of the object, and quenching the object which still being subjected to the alternating electromagnetic field automatically upon the change in the current producing the magnetic field incident to the disappearance of the magnetic properties of the material.

In testimony whereof, I have hereunto subscribed my name this twenty-ninth day of June, 1926.

WALTER JOHN BROWN. 

